Saurornitholestes

Foot of S. langstoni

In 1974 Canadian amateur paleontologist Irene Vanderloh discovered the skeleton of a small theropod near Steveville in Alberta. She showed it to John Storer of the Provincial Museum of Alberta, who brought it to the attention of Hans-Dieter Sues. In 1978 Sues named and described the specimen as the type species Saurornitholestes langstoni. The generic name is in reference to the Saurornithoididae, due to the resemblance with this group that is today seen as part of the Troodontidae, and combines their name with a Greek lestes, "thief". The specific name honours Wann Langston, Jr.

The holotype specimen, TMP 1974.10.5, was uncovered in a layer of the Dinosaur Park Formation dating to the late Campanian. It consists of a very fragmentary skeleton including teeth, skull elements, two vertebrae, ribs, tail elements and a part of the hand. Also three paratypes were assigned: CMN 12343, CMN 12354, and UA 5283, all frontals.[1]

Two more complete and larger partial skeletons (RTMP 88.121.39 and MOR 660), dozens of isolated bones, and scores of teeth are today known from the badlands of Dinosaur Provincial Park in Alberta; most of these are housed at the Royal Tyrrell Museum of Palaeontology, in Drumheller, Alberta and remain undescribed. The Alberta and Montana remains are usually attributed to the single species Saurornitholestes langstoni, though they come from a variety of rock formations indicating a wide span of time; for example, the Oldman Formation (dated to about 77 million years ago[2]) and the upper Two Medicine Formation (about 72 million years ago). Similar teeth are found in younger deposits, dated to around 70 to 69 million years ago,[3] but whether they represent S. langstoni or a different, related species is unknown. Neonate-sized Saurornitholestes fossils have been reported in the scientific literature.[4]

Fragmentary fossils of Saurornitholestes have also been found from the eastern half of North America. A tooth found in the Mooreville Chalk of Alabama has been assigned to the genus. In 2015, Schwimmer et al. identified the existence of Saurornitholestes langstoni from the Tar Heel, Coachman, and Donoho Creek formations of North and South Carolina based on diagnostic teeth and a pedal ungual. This also makes S. langstoni the only dromaeosaurid taxon reported with certainty from the East Coast.[5]

Saurornitholestes sullivani is known from the Hunter Wash fauna of the Kirtland Formation in New Mexico, based on the frontal SMP VP-1270. It differs from S. langstoni in the characters of the frontal.[6]

A well-preserved skeleton of Saurornitholestes discovered in 2014 is currently under preparation by University of Alberta paleontologists working in Japan.[7] After examining the skull of that specimen, Currie and Evans announced in 2019 that the Zapsalis teeth from the Dinosaur Park Formation represented the second premaxillary tooth of S. langstoni.[8]

Size of S. langstoni compared to a human

Like other theropods in the Dromaeosauridae, Saurornitholestes had a long, curving, blade-like claw on the second toe. Saurornitholestes was more long-legged and lightly built than other dromaeosaurids such as Velociraptor and Dromaeosaurus. It resembles Velociraptor in having large, fanglike teeth in the front of the jaws. Saurornitholestes most closely resembles Velociraptor, although the precise relationships of the Dromaeosauridae are still relatively poorly understood.

Saurornitholestes was about 1.8 m (5.9 ft) long[13] and weighed approximately 10 kg (22 lb).[14] At the hip it stood 0.6 meters (2 feet), or around as tall as the length of a terrier.

Saurornitholestes sullivani is thought to have had a keen sense of smell, due to its skull suggesting an unusually large olfactory bulb.[15]

The second premaxillary teeth of (at least) Saurornitholestes, Velociraptor, and Bambiraptor may have been structurally specialized for preening feathers. This may also have been the function of the unusual premaxillary teeth of the oviraptorosaurs Protarchaeopteryx and Incisivosaurus.[8]

In 1978 Sues assigned Saurornitholestes to the Dromaeosauridae. Later studies most often found it a member of the dromaeosaurid Velociraptorinae, but a cladistic analysis by Philip J. Currie in 2009 recovered a position in a more basal dromaeosaurid clade that was named the Saurornitholestinae.

The cladogram below follows a 2015 analysis by paleontologists Robert DePalma, David Burnham, Larry Martin, Peter Larson, and Robert Bakker, using updated data from the Theropod Working Group.[16]

Reconstructed S. langstoni skull

Another cladogram was recovered in a 2019 analysis by Philip J. Currie and David C. Evans. Currie and Evans recovered Saurornitholestes as the sister taxon of Atrociraptor.[8]

In 2001, Bruce Rothschild and others published a study examining evidence for stress fractures and tendon avulsions in theropod dinosaurs and the implications for their behavior. They found that only two of the 82 Saurornitholestes foot bones checked for stress fractures actually had them. Two of the nine hand bones examined for stress fractures were found to have them.[17]

Feeding habits

Restoration of an S. langstoni digging a multituberculate out of a burrow

Saurornitholestes' feeding habits were discovered to be typical of coelurosaurian theropods, with a characteristic "puncture and pull" feeding method. Studies of wear patterns on the teeth of this animal by Angelica Torices et al. in a study regarding theropod feeding habits indicate that dromaeosaurid teeth share similar wear patterns to those seen in the tyrannosauridae and troodontidae, respectively. However, micro-wear on the teeth indicated that Saurornitholestes likely preferred larger prey items than the troodontids it shared their environment with. Such differentiations in its diet likely allowed the theropod to inhabit the same environment as its more distant maniraptoran relations. The same study also indicated that both Saurornitholestes and the related Dromaeosaurus (also analyzed in the study) likely included bone in their diet and were better adapted to handle the stresses associated with attacking struggling prey while troodontids, equipped with weaker jaws, preyed on softer animals and prey items such as invertebrates and carrion. This feeding strategy and ability to handle struggling prey was also a feature that the dromaeosaurid also shared with tyrannosaurids such as Gorgosaurus, which was also analyzed in said study alongside these smaller theropods.[18][19]

Saurornitholestes was found on both sides of the Western Interior Seaway. Alberta, the location of Saurornitholestes langstoni, had a habitat similar to the United States Middle West being plains[20] and floodplain swamps.[21] In its eastern range, Saurornitholestes lived alongside hadrosaurs like Eotrachodon and Hypsibema, large theropods like Appalachiosaurus and Dryptosaurus, an unidentified ornithomimosaur, and another unidentified small theropod that was likely either a dromaeosaurid or a troodontid.[5]

Saurornitholestes appears to have been the most common small theropod in Dinosaur Provincial Park, and teeth and bones are much more common than those of its more massive contemporary, Dromaeosaurus. Little is known about what it ate and how it lived, but a tooth of Saurornitholestes has been found embedded in the wing bone of a large pterosaur, probably a juvenile Quetzalcoatlus.[22] Because the pterosaur was so much larger than Saurornitholestes, Currie and Jacobsen suggest that the theropod was probably scavenging the remains of an already dead animal.[22]

• Timeline of dromaeosaurid research

• Arbour, V.M.; Burns, M. E.; Sissons, R. L. (2009). "A redescription of the ankylosaurid dinosaur Dyoplosaurus acutosquameus Parks, 1924 (Ornithischia: Ankylosauria) and a revision of the genus". Journal of Vertebrate Paleontology. 29 (4): 1117–1135. doi:10.1671/039.029.0405.
• Currie, Philip J.; Jacobsen, Aase Roland (1995). "An azhdarchid pterosaur eaten by a velociraptorine theropod" (PDF). Canadian Journal of Earth Sciences. 32 (7): 922–925. doi:10.1139/e95-077. Archived from the original (PDF) on 2006-05-28.
• Currie P.J. & Koppelhus E.B., Dinosaur Provincial Park: a spectacular ancient ecosystem revealed, Vol. 1 (Indiana University Press, 2005), p. 372–373.
• Jacobsen, A.R. 2001. Tooth-marked small theropod bone: An extremely rare trace. p. 58-63. In: Mesozoic Vertebrate Life. Ed.s Tanke, D. H., Carpenter, K., Skrepnick, M. W. Indiana University Press.
• Longrich N.R. & Currie P.J. (2009), "A microraptorine (Dinosauria–Dromaeosauridae) from the Late Cretaceous of North America", PNAS 106(13): p. 5002-5007.
• Norell, Mark A. and Makovicky, Peter J. "Dromaeosauridae." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. Pp. 208. ISBN 0-520-24209-2
• Saurornitholestes - Dinosaur Saurornitholestes Characteristics, Behavior and Habitat; Subheadings: Habitat, Historical Period, Size and Weight
• Sullivan, R.M. (2006). "Saurornitholestes robustus, n. sp. (Theropoda:Dromaeosauridae) from the Upper Cretaceous Kirtland Formation (De-Na-Zin member_), San Juan Basin, New Mexico." NMMNH Bulletin 35: 253–256.
• Sullivan, R.M. and Lucas, S.G. (2006). "The Kirtlandian land-vertebrate "age" – faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America." New Mexico Museum of Natural History and Science, 35: 7-29.
• Tanke, D.H. and Brett-Surman, M.K. 2001. Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206–218. In: Mesozoic Vertebrate Life—New Research Inspired by the Paleontology of Philip J. Currie. Edited by D.H. Tanke and K. Carpenter. Indiana University Press: Bloomington. xviii + 577 pp.